The present invention relates to enhanced systems (methods and apparatus) for measurement or characterization in amplitude and phase of optical pulses using spectral phase interferometry for direct electric field reconstruction (SPIDER). More specifically, the invention uses temporally and/or temporally and spatially displaced phase differences which can be derived from a plurality of reference (homodyne) pulses and delayed replicas of the pulse to be measured, or from a plurality of spatially encoded, spectrally sheared replicas (SEA-SPIDER), or from time sheared and laterally separated replicas (space time or ST-SPIDER).
Ultrashort optical pulses now find wide application in many different fields including physics, chemistry, biology, materials processing and telecommunications, to name a few. Methods for characterization of the electric field of these pulses, i.e. the pulse shape in the time domain, have played a prominent role in advancing ultrafast technology. In many applications it is important both to optimize the pulse shape and to know the shape with reasonable accuracy, in order to interpret the results of an experiment. Such information is becoming increasingly important as the notion of control of nonlinear optical systems and processes (such as femtosecond laser machining and femtosecond laser tissue ablation) becomes more feasible.
It has been proposed to characterize the field of ultrashort optical pulses, but few of them have turned out to be useful practical solutions. Currently commercially available techniques include intensity autocorrelators, Frequency Resolved Optical Gating (FROG) and Spectral Phase Interferometry for Direct Electric-field Reconstruction (SPIDER). One of the inventors hereof is one of the inventors of SPIDER which is described in International Application No. PCT/US98/15355, published on Feb. 11, 1999 under International Publication No. WO99/06794, and U.S. patent application Ser. No. 09/463,918, filed Apr. 12, 2000 having priority to International Application No. PCT/US98/15355. Intensity autocorrelators were among the earliest tools used to measure the time evolution of short optical pulses. However, they only measure the temporal concentration of the energy, and not the shape of the electric field pulse itself, and do not therefore provide complete information about the pulse. FROG provides the pulse shape, but needs a lot of data and a sophisticated and intense numerical processing of these data to retrieve the pulse shape. SPIDER, because of its simple operating principles, needs far less data to retrieve the same information. Also, the processing is direct, which makes it very quick and error-free. The present invention uses and enhances SPIDER.
SPIDER as described in the above-referenced application provides a method and apparatus for measuring the electric field of ultrashort optical pulses using spectral shearing interferometer 13 (FIG. 1a). Its basic operating principle is as follows: two replicas T1 and T2 of the input test pulse 10 (i.e., the pulse one wishes to characterize) are delayed from each other by time xcfx84, and interact with a long chirped pulse 11 in a nonlinear crystal 12 (FIG. 1b). In a chirped pulse, the instantaneous frequency (i.e., the color) varies with time, so that each of the replicas T1 and T2 interacts with a different frequency in the chirped pulse (respectively xcfx890 and xcfx890+xcexa9). The nonlinear interaction shifts the mean frequency of each replica T1 and T2 by a different amount to provide the frequency-shifted replicas S1 and S2 which are then incident on a spectrometer 14, e.g., a one-dimensional (1-d) spectrometer. In the spectrometer, an array detector records the resulting spectral interferogram. The spectral phase of the input pulse 10 can be extracted from this interferogram using standard Fourier processing techniques. This quantity, when combined with the spectrum of the input pulse, completely characterizes the pulse. In this implementation, it is very important to calibrate properly the delay xcfx84 between the two initial pulses. Also, this delay sets a higher limit on the resolution of the spectrometer.
Briefly described, the present invention obtains and utilizes a plurality of spectral phase differences. A first embodiment applies the principle of homodyne detection to SPIDER. This dramatically improves the signal-to-noise ratio, thus enhancing the sensitivity of SPIDER. It also relaxes the constraints on the parameters used for the measurement. A second embodiment also allows one to relax the constraints on experimental parameters, and allows an easier implementation in some specific cases. It makes use of a different way of encoding and extracting the SPIDER signal. A third embodiment characterizes the pulse field at arbitrary transverse locations in the beam. While the electric field is a function of time only for the purpose of pulse measurement, actually, it is also a function of space. Differences in the field at different locations, which are very common in experiments using ultrashort optical pulses, can be very detrimental in applications. The present invention therefore provides for enhanced optical pulse measurement and thus may be used to achieve optimal output from ultrafast (e.g., subpicosecond) laser systems. Therefore, the invention has applications for the characterization of ultrabroadband pulses. The invention is useful for the study of the interaction of ultrashort optical pulses with matter.